Chemistry: electrical current producing apparatus – product – and – Plural concentric or single coiled electrode
Reexamination Certificate
2000-02-07
2002-04-23
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Plural concentric or single coiled electrode
C429S211000, C429S231950, C029S623100
Reexamination Certificate
active
06376121
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a lithium secondary cell and a method of manufacturing thereof, and more particularly to a lithium secondary cell in which a high power density is required and a method of manufacturing thereof.
BACKGROUND OF THE INVENTION
In recent years, cylindrical lithium ion secondary cells have been expected as a secondary cell that can achieve a large capacity and a high power density.
In such lithium secondary cells, the following drawbacks have been noted, particularly when such cells are required to have a large capacity and a high power density.
(1) The length of a sheet type electrode plate inevitably becomes large, and therefore, if the number of tabs is small, an effect of current collection declines. As a result, an internal resistance of the cell becomes large, and the cell performance is consequently deteriorated.
(2) In order to avoid unevenness of the potential distribution on the electrodes, it is required that the current collector tabs be evenly arranged in the longitudinal direction of the current collector. As a consequence, in order to avoid unevenness of reaction, it is required that each of the positive electrode active material and the negative electrode active material be present in a well-balanced manner on each opposite surface in each of the electrodes, with a separator interposed between the electrodes. It is to be noted here that the term the “unevenness of reaction” is intended to mean that when a negative electrode active material does not exist in a position in a negative electrode plate corresponding to a positive electrode active material, metal lithium is deposited because of the inhibition of the intercalation of lithium ions in the corresponding position.
In order to provide a solution for the above drawbacks, several types of lithium secondary cells have been suggested. One typical example of such a prior art cell is shown in a Japanese Unexamined Patent Publication No. 6-267528, which discloses a lithium secondary cell having the following construction. The cell has a sheet type current collector for both the positive and the negative electrodes, and in both electrodes, one lengthwise side part (i.e., both the upper and bottom portions of the spirally wound electrode assembly) is protruded from a separator with no active material applied thereon, so as to form a positive and a negative electrode lead connecting parts. A plurality of positive electrode leads are then connected to the positive electrode connecting parts, and a plurality of negative electrode leads are likewise connected to the negative electrode connecting parts.
The above prior art cell, however, has such drawbacks that, since the current collector tabs are welded to only one end of the current collector, the potential variation in the width direction of the electrodes is rendered large, that a large output power is not possible, and that a cycle life of the cell becomes short.
DISCLOSURE OF THE INVENTION
In view of the above-described drawbacks in prior art, it is an object of the present invention to provide a lithium secondary cell in which imbalance between the capacities of the positive electrode and the negative electrode is minimized, potential variation is reduced, and a high output power is achieved. The characteristics of the present invention are detailed in the following.
According to the present invention, there is provided a lithium secondary cell comprising a spirally-wound electrode assembly in which a positive electrode plate and a negative electrode plate are spirally wound with a sheet type separator interposed therebetween, the positive electrode plate comprising a sheet type positive electrode current collector and a positive electrode active material formed on both surfaces of the positive electrode current collector, and the negative electrode plate comprising a sheet type negative electrode current collector and a negative electrode active material formed on both surfaces of the negative electrode current collector, the lithium secondary cell characterized in that:
a plurality of positive electrode current collector tabs are attached to the positive electrode plate so that an interval is formed between the plurality of the positive electrode current collector tabs in a longitudinal direction of the positive electrode plate;
a plurality of negative electrode current collector tabs are attached to the negative electrode plate so that an interval is formed between the plurality of the negative electrode current collector tabs in a longitudinal direction of the negative electrode plate; and
in a state of the positive electrode plate and the negative electrode plate being wound, each of positive electrode current collector tabs and each of the negative electrode current collector tabs are so disposed as to face each other with the separator interposed therebetween.
According to the above construction, since the plurality of positive electrode current collector tabs and the plurality of the negative electrode current collector tabs are disposed so as to face each other with the separator interposed therebetween, the negative electrode active material is made to be present in a position corresponding to the positive electrode active material. As a result, deposition of metal lithium can be avoided, and a charge-discharge cycle life of the cell is improved. In addition, since the positive electrode current collector tabs and negative electrode current collector tabs are disposed so as to face each other in the longitudinal direction, unevenness of potential distribution caused in the longitudinal direction of the positive and the negative electrode plates can be thereby suppressed, and a cell with a high output power can be achieved.
Moreover, since the current collector tabs are provided in plurality in the longitudinal direction of the positive and the negative electrode plates, efficiency of current collection is thereby increased, and as a result, it is made possible to reduce an internal resistance of the cell and thereby improve the cell performance.
In addition, in the above construction of the lithium secondary cell, the following construction may be further employed. In the positive electrode plate, an applied area on which a positive electrode active material is applied and a non-applied area on which the positive electrode active material is not applied, are formed on the positive electrode plate alternately in the longitudinal direction of the positive electrode plate;
the positive electrode current collector tabs are firmly attached to the positive electrode current collector within non-applied area; and
a ratio W (L
4
/L
2
) of a width L
4
of the non-applied area to a width L
2
of the negative electrode current collector tab is between 1.2 and 3.5.
According to the above construction, because the aforementioned positive electrode current collector tabs are firmly fixed to the positive electrode current collector, it is made possible to realize a state in which the negative electrode current collector tabs are opposed to the positive electrode current collector tabs within the non-applied area in the electrode assembly being wounded together, even if a little displacement of the positive and the negative electrode plates occurs when winding the plates together. Accordingly, such an undesirable result is avoided that the positive electrode active material applied area and the negative electrode tab are overlapped, and therefore deposition of metal lithium is prevented.
The reason for restricting the ratio W to be between 1.2 and 3.5 are as follows. Firstly, if the ratio W is less than 1.2, there is a possibility of the negative electrode current collector tabs being overlapped with the positive electrode active material. On the other hand, if the ratio W is more than 3.5, the utilization factor of the positive electrode active material is reduced and thereby a high output power cannot be obtained.
In addition to the above-described constructions, the interval between the positive electrode current collecto
Inomata Hideyuki
Nakanishi Naoya
Nishio Koji
Nogami Mitsuzo
Yonezu Ikuo
Armstrong Westerman & Hattori, LLP
Brouillette Gabrielle
Sanyo Electric Co,. Ltd.
Tsang Susy
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